1. Field of the Invention
The present invention concerns a burner for a catalytic reactor, in particular a burner for use in secondary reformers.
2. Description of the Related Art
Burners for combustion of a reactant are mainly used for firing gas-fuelled industrial furnaces and process heaters, which require a stable flame with high combustion intensities. Such burners include a burner tube with a central tube for fuel supply surrounded by an oxidiser supply port. Intensive mixing of fuel and oxidiser in a combustion zone is achieved by passing the oxidiser through a swirler installed at the burner face on the central tube. The stream of oxidiser is, thereby, given a swirling-flow, which provides a high degree of internal and external recirculation of combustion products and a high combustion intensity.
More particular, burners for use in secondary reformers comprise burners in ammonia plants, where the methane reforming reaction from the tubular reformer is continued in the secondary reformer via the introduction of oxidant, i.e. air to the process stream for the reactor, hereby adding the nitrogen for the downstream ammonia loop and raising the temperature for the reforming process to take place in the secondary reformer catalyst bed, by combustion of the oxygen content. For this application a conventional burner is a nozzled ring burner. The nozzled ring type burner is equipped with specially designed nozzles installed on each of the air distribution holes, and seeks to achieve mixing at the burner nozzles, low metal temperatures of the burner, equal gas temperature distribution at the inlet to the catalyst bed and protection of the refractory lining from the hot flame core. Only a part of the process gas is combusted in the secondary reformer, whereas the remaining part flows further to the catalyst bed and to the steam reforming reaction.
The catalyst bed in the secondary reformer is covered with perforated refractory tiles in order to keep the catalyst in place. The very high temperatures in the secondary reformer cause the refractory tiles to slowly loose material by evaporation, and this material is later deposited by condensation in the catalyst bed below, where the temperature is dropping due to the heat consuming steam reforming reaction taking place here. The unwanted result is an increase in catalyst bed pressure drop, which eventually may lead to shut down of the plant in order to remove the deposited material.
The design of the burner is important to minimize the problem of catalyst bed pressure drop increase by the mechanism described above. Temperatures where process gas meets oxidant gas can locally rise to more than 2500° C., and it is very important to have good mixing downstream the point/points of initial contact between process gas and oxidant gas. Ideally all the process gas and combusted process gas are mixed to one mixture, with—the lowest possible—uniform temperature before the total gas flow reaches the layer of refractory tiles. This situation will give the lowest possible transport of material from refractory tiles to the catalyst bed. In comparison when a not fully mixed gas flow reaches the tiles there will be areas at lower temperatures and areas of higher temperatures than the uniform temperature. Compared to the situation of uniform temperature the situation with uneven temperatures causes a higher material loss from the tiles, because the transport mechanism accelerates dramatically by increasing temperature, and the increased material loss from hot areas therefore far outweigh the reduced material loss from cold areas.
A reduction in pressure drop over the burner on both the oxidant gas side and process gas side is often a benefit. When the pressure drop is reduced, it means that the maximum flow rate can be increased if the compression stage is the bottle neck of the plant. Some ammonia plants are running their oxidant gas compressor at maximum, and a decreased oxidant gas side pressure drop means that more oxidant gas can be supplied to the process gas stream. The process gas stream can be increased similarly to keep the ratio between nitrogen and hydrogen constant, and the effect is an increased ammonia production. If a flow increase is not of value, the reduced pressure drop will in most cases mean a cost reduction related to the reduction of compression energy needed.
A swirling burner for use in small and medium scale applications with substantially reduced internal recirculation of combustion products toward the burner face is disclosed in U.S. Pat. No. 5,496,170. The burner design disclosed in this patent results in a stable flame with high combustion intensity and without detrimental internal recirculation of hot combustion products by providing the burner with a swirling-flow of oxidiser having an overall flow direction concentrated along the axis of the combustion zone and at the same time directing the process gas flow towards the same axis. The disclosed swirling-flow burner comprises a burner tube and a central oxidiser supply tube concentric with and spaced from the burner tube, thereby defining an annular process gas channel between the tubes, the oxidiser supply tube and the process gas channel having separate inlet ends and separate outlet ends. U-shaped oxidiser and fuel gas injectors are arranged coaxial at the burner face. The burner is further equipped with a bluff body with static swirler blades extending inside the oxidiser injector. The swirler blades are mounted on the bluff body between their upstream end and their downstream end and extend to the surface of the oxidiser injection chamber.
US2002086257 discloses a swirling-flow burner with a burner tube comprising a central oxidiser supply tube and an outer concentric fuel supply tube, the oxidiser supply tube being provided with a concentric cylindrical guide body having static swirler blades and a central concentric cylindrical bore, the swirler blades extending from outer surface of the guide body to inner surface of oxidiser supply tube being concentrically arranged within space between the guide body and inner wall at lower portion of the oxidiser supply tube.
EP0685685 describes a gas injector nozzle comprising a discharge chamber with a cylindrical inner wall and having at its outlet end a circular gas discharge orifice, an outer wall concentrically surrounding the inner wall, the outer wall following a continuously curved path at a region at the chamber outlet end and being joint sharp-edged with the inner wall at the discharge orifice, wherein the curved path has a specific curvature radius.
Despite the above mentioned attempts to overcome the described problems related to burners, the burners of the known art design have been known to be challenged in cases where the operating conditions are particularly challenging.